Composting as a means to breakdown organic refuse has been known and practiced for probably thousands of years. Now that the problem of solid waste disposal has reached overwhelming proportions, all available methods are being explored and reevaluated. This is probably more the case with composting methods then any other, principally, because composting is a simple and environmentally safe means to process solid waste materials with the added advantage of a useful end product. In essence, composting is recycling, not merely burying or reducing the waste to toxic ash. We are just now seeing the beginning of municipal and industrial composting.
Composting is a natural process, the means by which turns raw organic waste into a useful material, one that can be mixed back into the soil to refurbish it with lost nutrients.
The composting process is an aerobic microbial decomposition of organic materials. Heat is generated during the process as well as carbon dioxide and water. When sufficient oxygen is present, odors are kept to a minimum and can easily be controlled. The microorganisms necessary for composting are usually present in the raw materials. Often, harmful microorganisms are also present, but these are eliminated by the heat generated during the decomposition. Careful temperature control of the composting mass is essential to attain the desired end.
Originally, composting was achieved by creating piles of compost material. As the method became more widely used long rows of composting material were found to be more efficient, since there could be better air circulation. This method, called windrows is still in use today, with modifications to make the process more efficient. An example can be seen in U.S. Pat. No. 4,230,676.
Horizontal composting tanks or containers have also been found to be effective. To these have been added mechanical means to mix or agitate the material during the composting period and to provide better inclusion of air and water when needed. The mixing also helps to evaporate excessive moisture. Such methods also provide better temperature regulation of the mass which in turn speeds the process. For example see U.S. Pat. No. 3,451,799. Means to improve fluid control through a composting mass were developed by Laurenson, Jr. (U.S. Pat. Nos. 4,837,153 and 4,410,349).
Johannsen, in U.S. Pat. No. 4,730,400 describes a drum reactor for aerobic fermentation having a rotary drum with air pipes along the outer shell whereby a plurality of jets deliver air and humidity along the length of the drum as it turns, thus insuring better circulation throughout the decomposing materal.
Sellew et al. in U.S. Pat. No. 4,869,877 teaches an elaborate and efficient system of bays in an enclosed building with a controlled ventilation system. Sensors actuate the system as the temperature in the composting mass reaches a preset value. This system can handle large quantities of municipal refuse in a continuous manner with new material introduced at a beginning point as the previously introduced material is moved along, finally to be removed automatically when the desired product is attained.
A method using modular containers is taught by Egarian (U.S. Pat. No. 4,956,002).
The method of Schiene et al. utilizes a set of orifices in the floor of the reaction chamber through which low intensity pressurized air is introduced. A ram and a surge of pressurized air are used to move the mass along through the composting chamber. (U.S. Pat. No. 5,023,178).
The newer horizontal systems have been found to be quite efficient, but require a large area of space for the systems to operate effectively. The silo or vertical chamber composting systems require much less ground area. The raw material must be raised to the top of the silo where it is introduced into the chamber through an opening or port. Thereafter, the material moves downward by gravity until the desired product is finally expelled at the bottom.
Early silo composting chambers were single chamber units lacking any regular agitation or control of the density of the mass or the air circulation throughout the decomposing material. Many of these had air introduced at the bottom of the tank whereby it rose up through the decomposing mass. Often highly pressurized air was required to force air upward through the settling mass. The introduction of air at the bottom of the chamber caused the highest temperatures to be at the bottom, where the material was ready to be removed, instead of at the top of the chamber where most of the decomposition should occur. It was not possible to control the upward flow of the air and if the mass became very dense, the upward flow of air could be severely reduced.
Kneer (U.S. Pat. Nos. 4,249,929; 4,184,269; and 4,062,770) utilizes a single chamber silo with probes distributed throughout the chamber at various levels to measure temperature and moisture content. Air is introduced at the bottom of the chamber, but the temperature and moisture content of the air are carefuly monitored. A suction device at the top draws the air upward and out of the chamber whereby it is passed over a heat exchanger and through a water separator. Finally, the exhaust air is passed through a biological filter before exiting the system.
Later silo units are divided into chambers or levels with means to agitate, aerate, and humidify or dehumidify at each level. Improvements in sensing devices and computerized controls enable careful monitoring and more exacting control over temperature and moisture content. Provisions are also made to recycle the heat generated in a reaction chamber to add to the efficiency of the system.
Pitwood (U.S. Pat. No. 3,756,784) developed a silo containing evenly spaced shelves which create a series of chambers each having air and water pipes and individually controlled centered paddles for agitation of the material. The air intake system brings air in from the outside if the pressure within drops, and exhaust valves vent the air to the outside if the pressure is too high or the reaction too fast. The uppermost chambers are also fitted with spray nozzles to provide water if needed. There is a transfer port in the floor of each chamber whereby the material is passed to the chamber below. The material remains in each of the seven chambers for 24 hours. Each chamber can be controlled separately to account for differences in the raw material.
Kaelin (U.S. Pat. No. 3,960,537) utilizes a one chamber silo with a central shaft on which are mounted a series of evenly spaced radial arms containing gas distributor blades. There are exhaust ducts built into the walls of the silo and the heated air is used to warm the new material being introduced into the top of the chamber.
In U.S. Pat. No. 4,358,540, Itoh et al. describe two embodiments of a multichamber silo composting apparatus. In one embodiment, a series of horizontal floors are attached to the circumference of the chamber and a central shaft contains arms which rotate within each chamber to agitate the material therein. The arms contain evenly spaced beaters which revolve as the arms sweep around the chamber. A single motor is mounted at the top of the shaft and provides the power for the central system. The speed of rotation can be varied as needed. In the second embodiment, the shelves are affixed to a central shaft which turns, and the arms with revolving beaters are affixed to one side of the silo. In both systems, the material is allowed to fall successively from one floor to the next below. Air supplied by a blower is admitted through holes in the floor of each chamber.
In all of the silo systems, the material in the reaction chamber or chambers moves downward by gravity. In many of the silo systems having more than one chamber there are no specific means used to control the passing of the mass from one level to the next lower level. As the material is being moved or swept around the floor of the chamber, it falls through the opening provided therein.